For internal combustion engines, such as diesel engines, nitrogen oxides (NOx) compounds may be emitted in the exhaust. To reduce NOx emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NOx compounds into more neutral compounds, such as diatomic nitrogen, water, or carbon dioxide, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant, such as anhydrous ammonia, aqueous ammonia, or urea is typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the SCR process, an SCR system may dose or otherwise introduce the reductant through a dosing module that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system upstream of the catalyst chamber.
Emissions systems for such compression-ignition (e.g., diesel) engines may monitor the release of carbon monoxide (CO), unburned hydrocarbons (UHC), diesel particulate matter (PM), such as ash and soot, and NOx. To monitor the release of such compounds, various sensors may be coupled to an exhaust system of the engine at various locations. For instance, NOx sensors may be coupled to the exhaust system downstream and/or upstream of a SCR system to monitor the amount of NOx upstream and/or downstream of the SCR system.
In some systems, diagnosis of a sensor consists of comparing a sensor reported value to a known reference value and identifying any low or high biases in the reported value. For exhaust systems with NOx sensors upstream and downstream of the SCR system, the reported values between the two sensors may show large differences due to the reductive activity of the NOx reducing catalyst of the SCR system and it may not be possible to compare the two sensors against each other to obtain a diagnosis. Accordingly, in some systems, an algorithmic diagnosis is performed by estimating the value of the NOx approaching the downstream sensor using mathematical models or by operating the system in a condition where the NOx converting catalyst has reduced activity.